1. Field of the Invention
This invention relates to a semiconductor device, and more particularly to a transistor having an additional carrier injecting junction formed adjacent to an emitter region.
2. Description of the Prior Art
A conventional transistor, which is obtained by a well known double-diffusion method, is to make an emitter region with higher impurity concentration than that of a base region in order to obtain a high amplification factor increasing an emitter efficiency, gamma (.gamma.).
A grounded-emitter current gain h.sub.FE, one of the parameters of the transistor characteristics is given by the following equation using a base-grounded amplification factor, alpha (.alpha.). ##EQU1## The amplification factor, alpha is given as EQU .alpha. = .alpha.* . .beta. . .gamma. 2
where .alpha.* is the collector multiplication ratio, .beta. is the base transport factor and .gamma. is the emitter efficiency.
In an NPN transistor, the emitter efficiency is given as follows: ##EQU2## Where J.sub.n is the current density of electrons injected from the emitter region into the base region, and J.sub.p is the current density of holes injected from the base region into the emitter region. Both current densities are given as follows: ##EQU3## Where L.sub.n is the diffusion length of electrons in the base region;
L.sub.p is the diffusion length of holes in the emitter region; PA1 D.sub.n is the electron diffusion constant; PA1 D.sub.p is the hole diffusion constant; PA1 N.sub.p is the minority carrier density in the base region; PA1 P.sub.n is the minority carrier density in the emitter region; and PA1 V is the applied voltage to the emitter-base junction.
The ratio P.sub.n /N.sub.p can be replaced by a ratio N.sub.A /N.sub.D when N.sub.D is the impurity concentration of the emitter region and N.sub.A is the impurity concentration of the base region. The diffusion length L.sub.n in the base region is limited by a base width W. As a result, the ratio .delta. is given as follows: ##EQU4## The diffusion constants are a function of the carrier mobility and the temperature and considered to be fixed.
It is clear that the ratio .delta. should be as small as possible in order to obtain a high emitter-grounded amplification factor h.sub.FE of the transistor.
In the conventional transistor made by the double-diffusion method, the impurity concentration N.sub.D in the emitter region is made high to reduce the ratio .delta.. However, the noise characteristics become bad when the impurity concentration in the emitter region is very high, for example, higher than about 10.sup.19 atoms/cm.sup.3, because the highly doped region includes lattice defects and dislocations therein, the emitter junction is poor.